The main issue is that prevailing winds have a direction, and there's not a continuous open ocean path other than the Southern Ocean which is harsh even by the standards of oceans. Sure you can steer along trade winds in the Atlantic or Pacific but there's quite some efficiency loss.
(Give climate change another decade or so and the Arctic Ocean maybe becomes an option, although by then we'll have bigger fish to fry, or perhaps poach).
Boats go considerably faster, as in overcoming more water drag, going crosswind than going downwind. I suppose that adding extra drag with a hydrogenerator will change the maths of that relationship, but certainly not so much that it would be prohibitively wasteful to not specialize a hydrogenerator carrier to downwind-only. (if the downwind-only setup can be competitive at all, not sure I'd take that as a given)
(Give climate change another decade or so and the Arctic Ocean maybe becomes an option, although by then we'll have bigger fish to fry, or perhaps poach).